Compostition: A good deck has an even balance between monsters and magic/traps, you generally want the composition of your deck around 50% monster, 25% magic and 25% trap. That way you are not constantly only drawing only one type of card. If the composition was 33% all 3 ways I would always be drawing magic and trap cards almost never monsters to defend yourself with or launch your offensive.

Drawing monsters: Only have a maximum of 4 monsters in your deck that require tribute to summon; includes ritual monsters excludes fusion monsters and synchro monsters. This keeps down how often you stuck with a high level creature in your hand versus a card to throw in defense mode and prolong the duel long enough to get the card you need.

Power VS the ease of summoning: On a similar note to above take into consideration how strong a monster is versus how hard is it to summon. Ex: Giga Gagagigo has 2450 atk and Elemental Hero Neos has 2500 atk. Neos requires 2 tributes and Gagagigo only requires 1, if I had to choose between which one to stick in my deck I would pick Gagagigo as he is easier to summon and only 50 points weaker. Sure you are taking an ever so slight hit on your attack force but he is easier to summon and can help you out of a tight spot easier then Neos can. I reccomend only using 1 tribute summon monsters unless the card has any easy work around to make summoning it easy. A good example is Blues Eyes white dragon; 2 tributes required 3000 atk and 2500 def. He is hard to summon but has alot of support cards for him, that make summoning him easy. So if I had those cards I would use him in a heart beat.

Pick a boss creature; this will be your most powerful card in your deck. My boss creature is Jinzo; he may be a bit weak on attack power compared to other people's boss creatures but his ability to diminish traps is very helpful.

Support: A boss creature not only needs to be the most powerful but needs alot of support from the rest of your deck. Ideally I would have machine specific cards for Jinzo to make him stronger and make him survive those tougher battles. Cards like machine production factory increase machine type cards attack stat. But since my deck has very few machines these cards are actually worthless. Cards that are generic in the type that they help are usually the best bet when using mixed decks. A good example is rising energy; it raises my creatures attack stat by 1500 until the end phase. This allows me to use offense defensively or lay a trap for my opponents stronger cards; making them think "Oh its an E-hero Sparkman; my Summoned Skull can take that on" thats when you spring Rising Energy to make Sparkman 600 points stronger.

Cards that are serious suggestions: Across all opponents
Note: Some of these come in the starter deck some don't.